The self-confessed mastermind of the 9/11 attacks and four co-defendants appeared in a Guantanamo Bay court on Saturday to be arraigned, all facing the death penalty if convicted.

Khalid Sheikh Mohammed and the other accused plotters -- appearing in public for the first time in three years -- challenged the court with small acts of defiance before being formally charged with planning and executing the September 11, 2001 attacks.

The suicide attacks by Al-Qaeda militants in hijacked airliners killed 2,976 people in New York, Washington and Shanksville, Pennsylvania.

"Prosecution is ready to proceed in the case of Khalid Sheikh Mohammed," chief prosecutor Brigadier General Mark Martins told the military tribunal.

The men, dressed in white jumpsuits and some wearing a white head covering, sat some distance from each other. Only one was handcuffed, while Mohammed sported a long, flowing beard.

Mohammed's lawyer David Nevin said his client probably would not speak at the hearing because he is "deeply concerned by the fairness of the process."

The judge, Colonel James Pohl, sought assurances that Mohammed was remaining silent by choice. Military authorities then censored the exchange that followed.

The lawyer for another of the accused, Walid bin Attash, then asked if cuffs could be taken off his client, saying he was "in pain." The judge ordered them taken off after receiving assurances Attash would "behave appropriately."

Ramzi Binalshibh, another of the accused, suddenly stood up and began to pray, interrupting the proceedings by alternately kneeling and rising.

One of the last steps before the so-called "trial of the century" takes place, the arraignment marks the second time the United States has tried to prosecute the 9/11 suspects.

It comes more than a decade after the most lethal attacks on US soil in modern history, and about one year after President Barack Obama ordered the US Navy SEALs raid that killed the man behind it all -- Osama bin Laden.

"There is a desire for justice. It is an important moment for all of us," said Marc Thiessen, a former speechwriter for president George W. Bush who has defended that administration's use of so-called enhanced interrogation techniques, which rights groups have denounced as torture.

Also in court at Guantanamo were Mohammed's Pakistani nephew Ali Abd al-Aziz Ali -- also known as Ammar al-Baluchi -- and Mustapha al-Hawsawi of Saudi Arabia.

The five have been held for years at the US naval base in southern Cuba while a legal and political battle has played out over how and where to prosecute them -- and debates have raged over their treatment.

It has been nine years since Mohammed's 2003 arrest, three of which he spent in secret CIA jails, confessing to a series of attacks and plots after being subjected to harsh interrogations, including waterboarding.

Some analysts say Mohammed could try to take advantage of the intense interest in the proceedings to deliver a scathing attack on the US government.

Out of 200 applicants, 60 journalists have obtained seats for the hearing, while another 30 are covering the event from Fort Meade in Maryland using a closed-circuit television feed.

"It's key to have transparency," Martins, the military commissions' chief prosecutor, told AFP.

In a sign of the acute public interest in the proceedings, the Pentagon has opened four military bases in the United States to allow families of the 9/11 victims to watch the case unfold on a giant screen.

Obama, a Democrat, wanted to hold the trial in a civilian court in Manhattan, just steps from Ground Zero, where the World Trade Center's Twin Towers once stood. But stiff Republican opposition in Congress scuttled those plans.

The military tribunal system, however, has come under scrutiny, and some of the sharpest criticism has come from former chief prosecutor Morris Davis.

"History will judge this as a mistake," he said.

The time that has lapsed since the attacks and the arrest of their presumed authors is a concern likely to come up at the hearing, said Ali's attorney James Connell.

The trial could still be years away, unless Mohammed pleads guilty to be put to death sooner and become a "martyr" for Al-Qaeda.

Cliff Russell is among the small number of victims' relatives who won a lottery to attend the arraignment proceedings. His firefighter brother was killed when the World Trade Center's twin towers collapsed in New York.

"I'm not looking forward to taking somebody's life... but it's the most disgusting, hateful awful thing I ever could think of; it's crazy," said Russell.